1. Field of the Invention
The present invention relates to a non-contacting electric potential sensor capable of being fabricated using micro-electro-mechanical-systems (MEMS) technology, and an image forming apparatus using the electric potential sensor.
2. Description of the Related Background Art
As a sensor for measuring a surface electric potential of an object to be measured (a measurement object), a variable capacitive coupling electric potential sensor of a mechanical type is known. FIG. 9 shows a principle of the mechanical type variable capacitive coupling potential sensor. A measurement object 1099 has an electric potential V relative to a ground potential. A detecting electrode 1021 is disposed facing the measurement object, and a movable shutter 1025 is arranged right above the detecting electrode 1021. Upon motion of the movable shutter 1025, an electrostatic capacitance C between the measurement object 1099 and the detecting electrode 1021 is changed. An electrical charge Q is induced in the detecting electrode 1021 in accordance with V and C. Current flowing between the detecting electrode 1021 and the ground is detected by an ampere meter 1060.
Here, the amount of the electrical charge Q induced in the detecting electrode 1021 satisfies a relation Q=CV. Accordingly, current i flowing into the ampere meter 1060 is given by i=dQ/dt=VdC/dt where t is the time. The potential V can be obtained if dC/dt is known. The sensitivity of the sensor is represented by dC/dt. As can be understood from that relation, the sensitivity increases if a difference between maximum and minimum of C is increased, or changing time t is shortened.
In connection with the above-discussed mechanical type variable capacitive coupling potential sensor capable of being fabricated using the MEMS technology, the following construction is disclosed by Japanese Patent Application Laid-Open No. 2000-147035 (its U.S. counterpart is U.S. Pat. No. 6,177,800). FIG. 10 illustrates an electric potential sensor 1001. The sensor 1001 is comprised of a driver component 1010 and a sensor component 1020. Those components can be fabricated on a substrate 1004 by the MEMS technology.
The driver component 1010 is comprised of a suspension 1018 having a parallel hinge structure, and a comb electrostatic actuator 1012. The comb electrostatic actuator 1012 is a general mechanism for driving a minute structure in an electrostatic manner, and is comprised of a movable electrode 1013 supported by the suspension 1018, and a stationary electrode 1014 attached to the substrate 1004. The comb electrostatic actuator 1012 is electrically connected to an electrostatic drive signal source 1050. The movable electrode 1013 is held by the suspension 1018 movably in right and left directions in FIG. 10. Comb-shaped electrodes of the movable electrode 1013 are in alternate mesh engagement with those of the stationary electrode 1014 with these two sets of Comb-shaped electrodes being spaced. Upon application of a potential difference between those two sets of Comb-shaped electrodes, electrostatic attractive force acts between those electrodes 1013 and 1014.
The sensor component 1020 is connected to the driver component 1010. A detecting electrode assembly 1021 is fixed to the substrate 1004, and is capable of capacitive coupling to a surface of an object to be measured. The detecting electrode assembly 1021 is composed of a set of spaced detecting electrodes 1021a, 1021b and 1021c. Those detecting probes are connected together such that individual signals can be combined or superimposed. The sensor component 1020 further includes a movable shutter 1025 which selectively covers the detecting electrode assembly 1021. The movable shutter 1025 is mechanically connected to the driver component 1010 such that a linear displacement of the driver component 1010 can cause a corresponding displacement of the movable shutter 1025.
The movable shutter 1025 has a plurality of openings 1024. The movable shutter 1025 is constructed such that the detecting electrode assembly 1021 can be selectively exposed through the openings 1024 when the movable shutter 1025 takes a first position. The openings 1024 are spaced from each other by a distance corresponding to the spacing between the detecting electrodes. When the movable shutter 1025 takes a second position, the detecting electrode assembly 1021 is covered with shielding portions 1026 present between the openings 1024.
In other words, when the movable shutter 1025 takes the first position, capacitive coupling between the detecting electrode assembly 1021 and the measurement object can be established. On the other hand, when the movable shutter 1025 takes the second position, capacitive coupling between the detecting electrode assembly 1021 and the measurement object is masked and prohibited. Current created by the detecting electrode assembly 1021 is output into a takeout electrode 1028, and is amplified by an amplifier 1060.
FIG. 11 is a cross-sectional view taken along a line 1080 of FIG. 10. As can be seen from FIG. 11, widths w1 of the respective detecting electrode of the detecting electrode assembly 1021 must be arranged with being spaced from each other by a distance w2 corresponding to the distance between the respective shutter openings 1024. Therefore, the width w1 is equal to the distance w2, and accordingly the effective area of the detecting electrodes is about a half of the area occupied thereby on the substrate.
As discussed above, the MEMS electrostatic sensor has the following disadvantages to be solved. In the first place, the driver component 1010 and the sensor component 1020 of the conventional MEMS electrostatic sensor are fabricated at different locations on the substrate 1004, respectively, and accordingly the chip size is liable to increase irrespective of their arrangement manner. Therefore, there is a limitation to reduction of the size of the conventional MEMS electrostatic sensor, and its cost increases.
Further, since the driver component 1010 and the sensor component 1020 move together, mass of a movable portion is likely to increase, and it is hence difficult to increase the driving frequency. The detecting sensitivity dC/dt of the electrostatic sensor is also proportional to the driving frequency, and accordingly the detecting sensitivity is difficult to increase.